feat: quality profile selection in desktop settings

Adds a Quality dropdown (Auto / Opus 24k / Opus 6k / Codec2 3.2k /
Codec2 1.2k) to both the connect screen and settings panel. The
selected profile is passed through to the engine which configures
the encoder and decoder accordingly.

The desktop engine recv path now auto-switches the decoder codec
when incoming packets use a different codec than expected, enabling
cross-codec interop between clients on different quality settings.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/index.html

@@ -26,6 +26,15 @@
           <label>Alias
             <input id="alias" type="text" placeholder="your name" />
           </label>
+          <label>Quality
+            <select id="quality">
+              <option value="auto">Auto</option>
+              <option value="good">Opus 24k</option>
+              <option value="degraded">Opus 6k</option>
+              <option value="codec2-3200">Codec2 3.2k</option>
+              <option value="catastrophic">Codec2 1.2k</option>
+            </select>
+          </label>
           <div class="form-row">
             <label class="checkbox">
               <input id="os-aec" type="checkbox" checked />
@@ -91,6 +100,15 @@
           </div>
           <div class="settings-section">
             <h3>Audio</h3>
+            <label>Quality
+              <select id="s-quality">
+                <option value="auto">Auto (adaptive)</option>
+                <option value="good">Good — Opus 24kbps</option>
+                <option value="degraded">Degraded — Opus 6kbps</option>
+                <option value="codec2-3200">Codec2 3200bps</option>
+                <option value="catastrophic">Catastrophic — Codec2 1200bps</option>
+              </select>
+            </label>
             <label class="checkbox">
               <input id="s-os-aec" type="checkbox" />
               OS Echo Cancellation (macOS VoiceProcessingIO)